Methods for cleaning substrates

ABSTRACT

A method for removing sand particles from a substrate is described. The method includes the step of treating the substrate with an acid solution comprising H x AF 6 , wherein A is selected from the group consisting of Si, Ge, Ti, Zr, Al, and Ga; and wherein H x AF 6  is present at a concentration in the range from about 5 weight percent to about 40 weight percent.

BACKGROUND

The invention relates to cleaning substrate surfaces, and moreparticularly to the removal of sand particles from turbine enginesurfaces, such as airfoils.

Sand fouling has recently emerged as a key factor significantlydegrading performance in turbine components. For example, aircraftengines flying domestic routes often experience significant sand foulingdue to heavy sand intake during flight idle, take-off, and landing. Ithas been determined that the primary mechanism for fouling is theincreased roughness of compressor blades due to sand ingestion.Specifically, this increased roughness results from the formation ofmicropits due to particle impact. Subsequently, sand particles withsizes less than 10 microns accumulate in these pits to form the foulinglayers. High temperatures in the downstream stages of the compressorresult in baking of the sand particles, which increases the airfoil-sandadhesion. Consequently, water wash treatments, which are frequently usedto clean the turbine components, often are not successful in removingthe accumulated sand particles.

Accordingly, there remains a need in the art for a method to remove sandparticles and sand-fouling layers from turbine engine components, forexample, while having minimal or no effect on the airfoil surfaceunderneath, or on any protective coatings on such a surface.

BRIEF DESCRIPTION

A primary embodiment of this invention is directed to a method forremoving sand particles from a substrate. The method includes the stepof treating the substrate with an acid solution comprising H_(x)AF₆,wherein A is selected from the group consisting of Si, Ge, Ti, Zr, Al,and Ga; and wherein H_(x)AF₆ is present at a concentration in the rangefrom about 5 weight percent to about 40 weight percent.

DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is a cross sectional view of a portion of a turbine enginecomponent; and

FIG. 2 illustrates a method for removing sand particles from asubstrate.

DETAILED DESCRIPTION

Embodiments of the present invention relate to a method for removingsand particles from a substrate. The method includes the step oftreating the substrate with an H_(x)AF₆ acid solution having aconcentration in the range from about 5 weight percent to about 40weight percent. As will be described in detail below, typically, milderprocessing conditions, such as lower concentrations of H_(x)AF₆ acid inthe aqueous solution, lower temperatures, and shorter treatment times,are preferred to remove the sand particles from the substrate withoutcausing substantial damage to the underlying coating.

As used herein, “sand” can generally be defined as a naturally occurringgranular material composed of finely divided rock and mineral particles.The most common constituent of sand is often silica (silicon dioxide, orSiO₂), typically in the form of quartz. However, the composition of sandcan vary widely. In some embodiments, the term “sand” is meant tospecifically refer to “CMAS” materials, which are materials based on amixture of calcia, magnesia, alumina and silica. The particle size ofthe sand particles is usually submicron. In one embodiment, the averageparticle size of the sand particles is in a range from about 0.5 micronto about 10 microns.

It has been found that sand fouling often takes place in aircraftengines flying in geographical locations that have desert areas.Airfoils in the compressor sections of such aircraft engines showsignificant accumulations of fine sand. Surfaces of the turbinecomponents, such as blades, can accumulate sand, resulting in reducedair flow, and a significant increase in specific fuel consumption (SFC).The accumulation of sand exists in both particulate-form, as well asglassy layers.

Sand particles with sizes less than 10 microns accumulate on theairfoils of the compressor. High temperatures in the downstream stagesof the compressor result in baking of the sand particles, whichincreases the airfoil-sand adhesion. The baking of the sand particlesforms a cement-like layer of the sand particles on the airfoil.Consequently, a water wash that is frequently effective in cleaning theturbine components is often not successful in removing the accumulatedsand particles.

The process includes treating the substrate with a mild aqueous solutionof H_(x)AF₆ acid at a selected temperature for a selected period oftime, and then rinsing the turbine engine component in an aqueousmedium. As used herein, the term “mild aqueous solution of H_(x)AF₆acid” refers to an aqueous solution of H_(x)AF₆ acid that issubstantially free of strong acids, such as phosphoric acid. The processof this invention employs fluosilicic acid or its derivatives, havingthe formula H_(x)AF₆. In this formula, A is selected from the groupconsisting of Si, Ge, Ti, Zr, Al, and Ga. The subscript “x” is aquantity from 1 to 6, and more typically, from 1 to 3. Materials of thistype are available commercially, or can be prepared without undueeffort. The preferred acid is H₂SiF₆. It is referred to by variousnames, such as “hydrofluosilicic acid”, “fluoro silicic acid”,“fluosilicic acid”, “hexafluorosilicic acid”, and “HFS”. Materials ofthis type are described in U.S. Pat. No. 6,599,416 (Kool et al), whichis incorporated herein by reference.

As described herein, the use of the H_(x)AF₆ compound providesconsiderable advantages for the sand-removal process. In many instances,such a process is very suitable, e.g., when there is a need for highremoval rates. Surprisingly it has been found that by employing theaqueous solution of fluosilicic acid and/or the derivatives of thefluosilicic acid, the sand particles can be removed from the surface ofthe airfoils without damaging the underlying metal alloy surface, or anyintervening coating, as described below.

Precursors to the H_(x)AF₆ acid may also be used. As used herein, a“precursor” refers to any compound or group of compounds which can becombined to form the acid or its dianion AF₆ ⁻², or which can betransformed into the acid or its dianion under reactive conditions, e.g.the action of heat, agitation, catalysts, and the like. Thus, the acidcan be formed, in situ, in a reaction vessel, for example.

As one illustration, the precursor may sometimes be a metal salt,inorganic salt, or an organic salt in which the dianion is ionicallybound. Non-limiting examples include salts of Ag, Na, Ni, and K, as wellas organic salts, such as a quaternary ammonium salt. Dissociation ofthe salts in an aqueous solution often yields the acid. In the case ofH₂SiF₆, a convenient salt which can be employed is Na₂SiF₆.

Those skilled in the art are familiar with the use of compounds whichcause the formation of H_(x)AF₆ within an aqueous composition. Forexample, H₂SiF₆ can be formed in situ by the reaction of asilicon-containing compound with a fluorine-containing compound. Anexemplary silicon-containing compound is SiO₂, while an exemplaryfluorine-containing compound is hydrofluoric acid (i.e., aqueoushydrogen fluoride).

Referring now to FIG. 1, there is depicted a cross-sectional view of aportion of an exemplary turbine engine component generally designated byreference numeral 10. The turbine engine component is an airfoil in thisinstance. The component 10 includes a concave surface 12 and a convexsurface 14. The sand particles can be removed from either surface, orfrom passageways, indentations, or various cavities in those surfaces.

The type of turbine engine component is not intended to be limited, andcan vary among a shroud, bucket or blade, nozzle or vane, diaphragmcomponent, seal component, valve stem, nozzle box, nozzle plate, or thelike. The terms “blade” and “bucket” can be used interchangeably.Generally a blade is a rotating airfoil of an aircraft turbine engine,and a bucket is a rotating airfoil of a land-based power generationturbine engine. Also the term “nozzle”, which generally refers to astationary vane in a steam or gas turbine, can be used interchangeablywith the term “vane”.

The turbine engine component, e.g., substrate 12, may generally comprisea variety of metallic alloys, e.g., steel or alloys thereof; or titaniumalloys (e.g., Ti-6Al-4V). In some preferred embodiments, the turbineengine component is formed from superalloys, which can be used at hightemperatures, often in excess of about 0.7 of the absolute meltingtemperature. The superalloys are based on nickel, cobalt, iron, or somecombination thereof. Many are described in the above-referenced patentto Kool et al.

As those skilled in the art understand, turbine engine components aremost often provided with one or more protective coatings. Therefore, thepresent invention contemplates the removal of sand from the surfaces ofthese coatings, as well as the removal of sand from an uncoatedsubstrate. Many different types of protective coatings are typicallyused, e.g., diffusion coatings or overlay coatings, as described in theKool patent. Moreover, overlay coatings like the “McrAl(X)” materials(considered to be metallic coatings) may be covered by thermal barriercoatings (TBC's). Non-limiting examples of the latter include ceramiccoatings such as yttria-stabilized zirconia. Some of the TBC's aredescribed in U.S. Pat. No. 6,921,586 (Zhao et al), which is incorporatedherein by reference. The thickness of the protective coating(s) willdepend on various factors, such as the type of article being coated, thecomposition of the substrate, and the environmental conditions to whichthe article will be subjected.

FIG. 2 is an illustrative flow chart for an exemplary, non-limitingmethod of removing sand particles from a substrate. At block 20, thesubstrate is treated with an H_(x)AF₆ acid solution having aconcentration in the range from about 5 weight percent to about 40weight percent. For example, the substrate can be treated by at leastpartially immersing the substrate in a bath of the H_(x)AF₆ acidsolution. The bath may be mechanically agitated during treatment.

The preferred level of H_(x)AF₆ acid which is employed will depend onvarious factors. They include the type and amount of sand being removed;the location of the sand on a substrate; the type of substrate; thethermal history of the substrate and sand material (e.g., how tightlyadherent the sand is to the substrate); the time and temperature usedfor treatment; and the stability of the acid in the treatment solution.

As a general rule, the H_(x)AF₆ acid is present in a treatmentcomposition at a level in the range of about 5 weight percent to about40 weight percent. Usually, the level is in the range of about 10 weightpercent to about 30 weight percent. In the case of H₂SiF₆, a preferredconcentration (here in molar units) is about 0.2M to about 2.2M.Adjustment of the amount of H_(x)AF₆ acid, and of other componentsdescribed below, can readily be made by considering stoichiometricparameters, and by observing the effect of particular compositions onsand-removal from the substrate.

The temperature of the H_(x)AF₆ acid solution can usually be maintainedup to about 100° C. In some specific embodiments, the temperature ismaintained below about 50° C. In some especially preferred embodiments,the temperature range is from about 50° C. to about 80° C. Typically,ambient pressure is suitable while treating the substrate with theH_(x)AF₆ acid.

The time required for removing the sand particles (i.e., the immersiontime within the aqueous composition) may vary considerably. Factorswhich influence the selection of an appropriate time include theparticular composition of the sand-based material being removed; as wellas its density and thickness. The time may also vary with thetemperature of the H_(x)AF₆ acid solution. For example, for a highertemperature of the fluosilicic acid solution, the treatment time may beshorter, and vice versa. Usually, the substrate is at least partiallyimmersed in a bath of the wetting agent for treatment, for a period oftime in a range from about 1 minute to about 36 hours, and preferably,from about 5 minutes to about 8 hours. In some instances, an especiallypreferred immersion time is in the range of about 10 minutes to about 2hours. The immersion time for the substrate in the bath usually dependson the temperature of the bath and the type of protective coating on thesubstrate. For example, longer immersion times are acceptable forsubstrates having protective coatings, such as MCrAlY and TBC, becauseof the inert nature of such protective coatings towards the H_(x)AF₆acid solution. Accordingly, in the case of such protective coatings, theunderlying substrate remains mostly unharmed—even after prolongedimmersion of the substrate in the H_(x)AF₆ acid solution.

The aqueous composition used for the present invention may includevarious other additives that serve a variety of functions. Non-limitingexamples of these additives are inhibitors, dispersants, surfactants,chelating agents, wetting agents, deflocculants, stabilizers,anti-settling agents, and anti-foam agents. Those of ordinary skill inthe art are familiar with specific types of such additives, and witheffective levels for their use. An example of an inhibitor for thecomposition is a relatively weak acid like acetic acid, mentioned above.Such a material tends to lower the activity of the primary acid in thecomposition. This is desirable in some instances, e.g., to decrease thepossibility of any damage to the underlying coating(s) or substratesurface.

The aqueous solution of the H_(x)AF₆ acid of the present invention isnot only effective at removing the deposit containing the sand particleson the substrate, but is also usually benign to the underlying metal ofthe airfoil, and to any of the protective coatings. In other words, withproper monitoring of the treatment process, the underlying surfaces arenot adversely affected.

Optionally, as illustrated in block 22, the substrate is treated with awetting agent, such as a surfactant, prior to treatment with theH_(x)AF₆ acid solution. The treatment with the wetting agent canfacilitate better interaction between the sand particles and theH_(x)AF₆ acid to facilitate removal of the coating.

Non-limiting examples of suitable surfactants include detergents, acidstable surfactants, non-acid stable surfactants, or combinationsthereof. An example of an acid-stable surfactant may includepolyethylene oxide, which is commercially available as Triton x-100™(manufactured by Research Chemicals Limited [CAS No. 9002-93-1]). Anexample of non-acid stable surfactant includes trisodiumphosphate, whichis commercially available as Alconox TSP™

Optionally, as illustrated in block 24, the substrate is rinsed withwater, prior to treatment with the wetting agent. Rinsing the substratewith water removes any dirt particles or loose sand particles from thesurface of the substrate.

Optionally, at block 26, the substrate may be rinsed with water oranother liquid after treatment with the H_(x)AF₆ acid solution, toremove any debris or undesired chemicals from the surface of thesubstrate. In one embodiment, rinsing may be done by applying a waterjet.

In addition to (or in lieu of) the rinsing step 26, treatment of thesand-fouled surface may also include at least one subsequent step, toremove any remaining residue—sand or otherwise. As described in thereferenced Kool patent, a post-cleaning or “de-smutting” operation canbe undertaken. It may be in the form of an abrasion step which minimizesdamage to the substrate or protective coatings. As one example, agrit-blasting can be carried out by directing a pressurized air streamcontaining aluminum oxide particles across the surface. The air pressureis usually less than about 100 psi. The grit-blasting is carried out fora time period sufficient to remove any residual material. Other knowntechniques for abrading the surface may be used in lieu ofgrit-blasting. Many of these are described in U.S. Pat. No. 5,976,265,incorporated herein by reference. For example, the surface can bemanually scrubbed with a fiber pad, e.g. a pad with polymeric, metallic,or ceramic fibers. Alternatively, the surface can be polished with aflexible wheel or belt in which alumina or silicon carbide particleshave been embedded. Liquid abrasive materials may alternatively be usedon the wheels or belts. These alternative techniques would be controlledin a manner that maintained a contact force against the surface that wasno greater than the force used in the grit-blasting technique discussedabove.

Other techniques (or combinations of techniques) could be employed inplace of abrasion, to remove any residual material. Examples includetumbling of the article (e.g., water-tumbling), or laser ablation of itssurface. Alternatively, the residual material could be scraped off thesurface. As still another alternative, sound waves (e.g., ultrasonic)could be directed against the surface, causing vibrations which canshake loose the degraded material. For each of these alternativetechniques, those skilled in the art would be familiar with operatingadjustments which are made to control the relevant force applied againstthe surface of the article (as in the case of the abrasion technique),to minimize damage to the substrate or overlying protective coatings.The article is sometimes rinsed after this step, e.g., using water or acombination of water and a wetting agent.

The bath may be mechanically agitated during this residue-removal step.For example, the bath may be mechanically agitated (as describedpreviously) while treating the substrate in the H_(x)AF₆ acid solution.In one embodiment, the bath may be mechanically agitated while treatingthe substrate with a wetting agent.

As mentioned previously, the present invention can also be used toefficiently remove sand from any internal region or cavity in anarticle, e.g., indentations, hollow regions, passages, or holes. In thecase of a turbine airfoil, the internal region is often in the form ofradial cooling holes or serpentine passageways. The process of thepresent technique produces a synergistic effect, and results in anextremely effective cleaning method for airfoils and other turbineengine parts which contain tightly-adherent sand particles.

While only certain features of the invention have been illustrated anddescribed herein, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the scope of the invention.

1. A method for removing sand particles from a substrate, comprising thestep of treating the substrate with an acid solution comprisingH_(x)AF₆, wherein A is selected from the group consisting of Si, Ge, Ti,Zr, Al, and Ga; and wherein H_(x)AF₆ is present at a concentration inthe range from about 5 weight percent to about 40 weight percent.
 2. Themethod of claim 1, wherein the H_(x)AF₆ acid solution is an aqueoussolution.
 3. The method of claim 1, wherein the concentration of theH_(x)AF₆ acid is in a range from about 10 weight percent to about 30weight percent.
 4. The method of claim 2, wherein the H_(x)AF₆ acidsolution is maintained at a temperature no greater than about 100° C.during treatment.
 5. The method of claim 1, wherein the substrate istreated with a wetting agent, prior to treatment with the fluosilicicacid solution.
 6. The method of claim 5, wherein the wetting agentcomprises a surfactant.
 7. The method of claim 5, further comprisingrinsing the substrate with water prior to treatment with the wettingagent.
 8. The method of claim 1, further comprising rinsing thesubstrate with water after treatment with the H_(x)AF₆ acid solution. 9.The method of claim 1, wherein the treatment with the H_(x)AF₆ acidsolution is carried out for a time period in a range from about 10minutes to about 2 hours.
 10. The method of claim 1, wherein treatmentwith the H_(x)AF₆ acid solution is carried out at a temperature in arange from about 20 degrees Celsius to about 100 degrees Celsius. 11.The method of claim 10, wherein the temperature is in a range from about50 degrees Celsius to about 80 degrees Celsius.
 12. The method of claim1, wherein the substrate is at least partially immersed in a bath of theH_(x)AF₆ acid solution during treatment.
 13. The method of claim 12,wherein the bath is mechanically agitated during treatment.
 14. Themethod of claim 5, wherein the substrate is at least partially immersedin a bath of the wetting agent for treatment, for a period of time in arange from about 10 minutes to about 2 hours.
 15. The method of claim 1,wherein the substrate is a section of a turbine engine component. 16.The method of claim 15, wherein the turbine engine component is anairfoil.
 17. The method of claim 1, wherein the sand particles have anaverage particle size in the range of about 0.5 micron to about 10microns.
 18. The method of claim 15, wherein the turbine enginecomponent is formed of a material which comprises a steel, a superalloy,a titanium alloy, or a combination thereof.
 19. The method of claim 18,wherein the turbine engine component comprises a shroud, a bucket, ablade, a nozzle, a vane, a diaphragm component, a seal component, or avalve stem.
 20. The method of claim 18, wherein the coating comprisesCMAS (calcium-magnesia-alumina-silicate).
 21. The method of claim 1,wherein at least one protective coating is disposed over the substrate;and the sand particles being removed are those which adhere to a surfaceof the protective coating.
 22. The method of claim 21, wherein theprotective coating comprises a diffusion coating or an overlay coating.23. The method of claim 21, wherein the protective coating comprises anunderlying metallic coating, and an overlying ceramic barrier coating.24. A method for removing sand particles from a surface of at least oneprotective coating disposed on a superalloy substrate, comprising thestep of treating the coated substrate with an acid solution comprisingH_(x)AF₆, wherein A is selected from the group consisting of Si, Ge, Ti,Zr, Al, and Ga; and wherein H_(x)AF₆ is present at a concentration inthe range from about 5 weight percent to about 40 weight percent. 25.The method of claim 24, wherein the protective coating is a thermalbarrier coating.